1. Field of the Invention
The present invention generally relates to a battery pack capacity adjusting device and method for adjusting a capacity of a battery pack having a plurality of secondary cells.
2. Background Information
When a battery pack having a plurality of cells (secondary cells) connected together is charged and discharged repeatedly and/or left unused for a period of time, capacity differences develop among the cells due to variation in the characteristics of the cells. If the battery pack is used in a state in which such capacity differences exist, there will be some cells that become overcharged or overdischarged, and the service life of the battery pack as a whole will be shortened. Therefore, the capacities of the individual cells are adjusted at a prescribed frequency to make them substantially uniform.
Organic solvents, such as ethylene carbonate, are used as electrolytes in lithium based secondary batteries such as lithium ion secondary batteries that have a positive electrode made of lithium cobalt oxide and a negative electrode made of carbon, and lithium secondary batteries that have a positive electrode made of lithium cobalt oxide and a negative electrode made of lithium metal. When a lithium based secondary battery is overcharged, the organic solvent decomposes and vaporizes, causing the encasement of the secondary battery to bulge abnormally. Additionally, since the organic solvent serving as the electrolyte evaporates, the charging capacity declines greatly the next time the secondary battery is charged.
Consequently, a method is employed with lithium based secondary batteries whereby the capacities of the individual cells are adjusted to be uniform by discharging the cells that have a larger capacity than the others. This capacity adjustment of the individual cells is accomplished by discharging the cells through bypass resistors connected in parallel with each of the cells for an amount of time corresponding to the capacity adjustment.
However, when capacity adjustment discharging is executed through a large number of the capacity adjustment bypass resistors, the amount of emitted heat from the resistors can become excessive. Thus, there is the possibility that a CPU and other electronic components coupled to the bypass resistor mounted on the control circuit board will be adversely affected by the heat. In order to address this issue, Japanese Laid-Open Patent Application Publication No. 2006-73364 proposes prohibiting the capacity adjustment of those cells whose charge capacities are not excessively uneven when the temperature of the circuit board, on which the capacity adjustment bypass resistors are mounted, exceeds a threshold value.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved battery pack capacity adjusting device and method. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.